2005 Other Mercedes Benz Models Car alarm keeps going off

The alarm horn has its own internal battery and a 12v reference going to it via wires. If it does not see that 12v ref because of a defective alarm horn, poor contact, or open wire it will sound because it thinks it is being tampered with.

The other possibility is a door/trunk/hood contact that goes open intermittently, causing the alarm to sound.

The dealer has the ability to see what the cause of last few alarm activations were using Star Diagnosis, without it diagnosis is pretty much impossible.